1. Field of the Invention
The present invention relates to a semiconductor device in which copper wire is used as a bonding wire and, more particularly, to improvements in the structure of electrode pads of a semiconductor device.
2. Description of the Prior Art
Heretofore, in a semiconductor device, copper or copper alloy is used as a bonding wire, and aluminum or aluminum alloy is used as an electrode pad, which is also known as bonding electrode. FIG. 1 is a sectional view of such a conventional semiconductor device which has yet to be wire-bonded. FIG. 2 is a sectional view of the semiconductor device which has been wire-bonded. In these figures, reference numeral 11 denotes a lead frame. Semiconductor element 13 is bonded to the lead frame by means of conductive paste 12, for example, silver paste, on the lead frame. Insulating films 14 are formed at predetermined positions on semiconductor element 13. Electrode pad 15, formed of a layer of aluminum or aluminum alloy, is formed on those portions of the semiconductor element which are not covered by the insulating films. As is shown in FIG. 2, copper bonding wire 16 is bonded to electrode pad 15. The electrode pad is deformed by the pressure of bonding the hard bonding wire to the electrode pad so that the bonding wire may contact semiconductor element 13 through the electrode pad as shown in FIG. 2. The direct contact of the wire with the element will alter the electric characteristics of the bonding portion, and mortally vary to malfunction the electric characteristics of the semiconductor device exposed at high temperature. Since the hard wire is pressed directly on semiconductor element 13, element 13 is distorted. Temperature changes, i.e., temperature cycles, cause the semiconductor element to have cracks, due to the difference in thermal expansion coefficients between the semiconductor element and the wire. The electrode pad is more easily corroded than other portions during patterning process, thus exposing the semiconductor element. Thus, the semiconductor device is liable to malfunction during use.